1. Field of the Invention
The present invention relates to a controlled depth laser drilling system which utilizes optical means to transmit original laser beam radiation, and transmit laser beam radiation reflected from a workpiece, to a radiation detector. This detector is capable of converting the reflected radiation to measurable electrical signals.
2. Description of the Prior Art
Printed circuit boards having a plurality of layers and containing top and embedded copper or other type metal foil, and electrically conducting vias with plated through-holes and blind-holes are well known, and taught, for example by Marchetti et al., in U.S. Pat. No. 4,501,787. In the past, such through holes and blind holes have been mechanically drilled. However, even the most modern mechanical microdrilling technology, for example that taught by Saxton et al., in U.S. Pat. No. 4,536,108, cannot provide drilled holes smaller than about 4 mil (0.004 inch). Such holes are drilled by miniature drill bits in conjunction with depth sensors. Such miniature drills sustain significant cutting edge damage when drilled through some modern circuit board substrates, such as epoxy resin impregnated glass cloth, and therefore have a limited life. Such miniature drills are also expensive and subject to a wide variety of torque, thrust, friction and deflection forces, resulting in a high rate of microdrill breakage. Also, existing equipment using miniature drills is designed to drill boards in three board stacks at a rate of about 200 inches per minute. Boards requiring microholes cannot be stacked, and the feed rate drops to about 50 inches per minute, an overall efficiency loss of about 12/1.
Mader, in U.S. Pat. No. 4,240,094, used a variety of laser beam deflecting optics, a final objective lens, and a viewing camera, operating through a partially reflective, partially transmissive mirror, to cause a laser beam to selectively disconnect semiconductor connection paths, and to interconnect metalization connection paths to selected semiconductor materials, on the top surface of a large scale integrated circuit module. In 1977 Kestenbaum, in U.S. Pat. No. 4,044,222, taught short-pulsed, tapered aperture formation in thin films, such as silicon dioxide, silicon nitride and tantalum oxides, using a Q-switched CO.sub.2 laser, or a cavity dumped or mode-locked YAG (yttrium aluminum garnet) laser. These films, about 400 Angstrom Units to about 10,000 Angstrom Units thick, were usually deposits on semiconductor substrates.
From about 1980, experimental CO.sub.2 laser drilling of circuit boards expanded. W. Wrenner, in "Generating Small Holes For IBM's New LSI Package Design", IPC-TP-446, IPC Fall Meeting, October 1982, generally describes plated, laser drilled blind and through-hole technology for advanced circuit board package designs. A pulsed CO.sub.2 laser was used to drill epoxy glass substrates through a copper mask. Use of a single laser to drill multiple parts was also described. L. Fenichel, in Circuits Manufacturing, Vol. 24, February 1984, pp. 49-54, describes two commercial CO.sub.2 laser drilling systems useful for drilling blind holes in copper clad, epoxy-glass laminates through a copper mask. Neither of these articles goes into great detail about the laser optics systems.
Toida et al., in U.S. Pat. Nos. 4,532,400 and 4,550,240, teach use of dual lasers, various reflector means, and articulated arm and fiber optic beam guides to allow surgical cutting of tissue by a CO.sub.2 laser beam and coagulation of blood by a YAG laser beam. There, a He--Ne laser is used to output a visible, red first guide beam, and a halogen lamp is used to output a visible, white second guide beam, which beams are used in conjunction with invisible CO.sub.2 and YAG laser beams. The visible beams are mixed with the invisible CO.sub.2 and YAG beams, by an assembly of reflecting and dichroic mirrors, to provide the output beams. Four operating modes are taught: coaxial irradiation of CO.sub.2 and YAG laser beams with the He--Ne visible red beam through an articulated arm light guide; irradiation of the CO.sub.2 laser beam with the He--Ne visible red beam through an articulated arm light guide; irradiation of the YAG laser beam with the He--Ne visible red beam through an articulated arm light guide; and irradiation of the YAG laser beam with the halogen lamp visible white beam through a fiber optic light guide.
Lassen, in U.S. Pat. No. 4,544,442, teaches pulsed laser and other type drilling of 6 mil to 12 mil diameter holes in organic, compact microelectronic substrate packages, having round and foil conductors disposed in a pattern therein. The laser drilling provides access to the round, or both the round and the foil conductors. The drilling is followed by metal plating of the hole and exposed conductors. The preferred laser is a CO.sub.2 laser applied vertically to the workpiece through a mirror and mirror head, when metal conductors are used. Such drilling is achieved, by in some fashion using the contrast between the reflective power of the metallic conductors to a CO.sub.2 laser beam, and the absorptive power of the organic substrate. The organic material could also be removed with other type lasers, a controlled depth mechanical drill, a modulated stream of abrasive particles or a water jet stream of chemicals. The problem with the Lassen system is that one would never be quite sure of hole penetration depth if laser beams are used, so that if small round conductors are used, the hole may be drilled substantially beyond the conductor.
Melcher et al., in U.S. Pat. No. 4,504,727, teach laser drilling of multilayer printed circuits. They monitor drilling depth by utilizng a real time, photoacoustic feedback control. Melcher et al. teach use of solid state photoacoustic spectroscopy, where light energy is absorbed by a solid, which energy is converted into an acoustic signal which is characteristic of that solid, and then converted into an electrical signal for analysis. The primary source of the acoustic signal arises from the periodic, time-dependent heat flow, caused by laser vaporization of solids, from the solid to the surrounding gas. The acoustic signal is typically detected by a microphone. The lasers suggested were a Nd/YAG laser or a CO.sub.2 laser. This type of feedback control monitors and amplifies a noise-free signal from the drilling laser via a laser beam splitter and infrared radiation sensor combination, and also a signature, noise containing signal from a piezoelectric sensor or microphone close to the workpiece, which signal must be filtered and amplified. Such a system is very complicated, requiring very sophisticated acoustic monitors and filters. What is needed is a less expensive, less complicated, purely optical, drill depth control system.